J. Phys. Chem. 1984, 88, 2303-2307
2303
Flexibility of the Framework of Zeolite Rho. Structural Variation from 11 to 573 K. A Study Using Neutron Powder Diffraction Data John B. Parise,? Lloyd Abrams,* Thurmond E. Gier, David R. Corbin, Central Research and Development Department,$ Experimental Station, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19898
James D. Jorgensen, Materials Science and Technology Division, Argonne National Laboratory, Argonne, Illinois 60616
and Edward Prince National Measurements Laboratory, National Bureau of Standards, Washington, D.C. 20234 (Received: July 7, 1983; In Final Form: October 31, 1983)
The structure of the dehydrated and deuterium-exchanged form of deammoniated zeolite Rho (Csl 15(SiA1)48096D) has been studied at 11, 295, 423, and 573 K. All data sets were refined by using the Rietveld refinement technique in the noncentrosymmetric space group I43m and are characterized by the presence of elliptically distorted double 8-rings. In agreement with a previous study of the effects of hydration upon the framework, increase in temperature causes a monotonic increase in the cubic-unit-cell parameter (ao)and a corresponding decrease in A, the parameter describing the “degree of ellipticity” as measured by the difference between the major and minor axes of the 8-ring ellipses. The variations of both A and a. with temperature in the range 11 < T < 573 K appear to fall close to smooth curves. Extrapolation of these curves suggests that the structure may be centrosymmetric above 800 K. This prediction is supported by a study of the changes in the X-ray diffraction pattern of deammoniated Rho up to 7 7 3 K.
Introduction The relationship between the dimensions and arrangements of regularly spaced pores and channels of crystalline zeolites and their shape-selective absorptive and catalytic properties is wellknown and much Although variation of the sizes and shapes of the pores as a function of temperature is acknowledged: the interpretation of properties is usually based on structural analysis performed on data sets collected on hydrated or dehydrated samples at room temperature. To gain a better understanding of the dynamics and flexibility of the zeolite framework, computer modeling and experimental studies of the changes occurring upon dehydration of the cubic zeolite, R ~ o ,have ~ , ~proven useful. In this case, a simple distortion involving the “breathing” of a double 8-ring (Figure 1 ) can be modeled as a function of unit-cell dimension (ao). The degree to which the ring opening will distort is measured by a parameter A (Figure 1 ) . This distortion is accompanied by a large change in cell dimension, and the variation of A with a can be p r e d i ~ t e d .Previous ~ has focused on the variation of A without regard to which particular variables (ion exchange, hydration, temperature, or a combination thereof) caused the distortion. It is the purpose of this paper to present data on the flexibility of the framework of zeolite Rho as a function of temperature. It was necessary to ensure that all samples had the same chemical composition, were calcined under the same conditions, and were thoroughly deuterated and dehydrated. Once a variation as a function of temperature had been established, it was hoped that this might lead to more unambiguous interpretations of the effects of ion exchange and hydration. The structural refinements discussed below were performed by using the Riet~eld’-~ refinement technique. In this procedure, the whole pattern is fitted by simultaneously varying atomic structural parameters (position and thermal) and profile parameters (cell dimensions, half-widths, and diffractometer zero, for example). There is no intermediate step to extract structure factors and the sd where ylca’cdand function minimized is x2 = ~ l w , 2 C y ~-by,ca1cd)2 ypb”lare the calculated and observed profile intensities at position ‘Current adress: Research School of Chemistry, Australian National University, Canberra, ACT 2600, Australia. *Contribution 3275.
0022-3654/84/2088-2303$01.50/0
i , and w, is the weight assigned each observation and equals and y,Obsdfor D-Rho1 1K are provided l/(ypbd). (Values of yJcalcd in a supplementary table. See paragraph at end of text regarding supplementary material.) Background is not subtracted prior to refinement and is included as a variable in the least-squares procedure. This technique has been tested extensively and allows the greatest amount of information to be derived from a powder diffraction data set free from systematic error.
Experimental Section Sample Preparation. The preparation of the hydrogen form of zeolite Rho has been described el~ewhere.~ Samples from the same batch were all exchanged in ammonium chloride and then calcined at 873 K for 16 h and at 923 K for 1 h in flowing argon. Chemical analysis gave a unit-cell formula of Cs, zE+9lSi37,AI,, Extra framework cations (E+) required for charge balance were presumed to be H+, H 3 0 + , or NH4+ species. Because the samples were to be used for neutron diffraction investigations, they were thoroughly exchanged in D20 and dehydrated prior to data collection to decrease the contribution to the background due to incoherent scattering from hydrogen. Samples were placed in Pyrex glass containers on a standard vacuum rack, heated to 640 K for 13 h, cooled to 450 K, and then exposed to DzO at 5 torr. This process was repeated 3 times before the samples were finally dehydrated at 640 K for 16 h and sealed under vacuum of better than 1 mtorr. The powders of Rho were loaded into either aluminum or vanadium sample containers for data collection at either the National Bureau of Standards (NBS) reactor or the Intense Pulsed Neutron Source (IPNS) at Argonne (I) (2) (3) (4)
Breck, D. W. “Zeolite Molecular Sieves”; Wiley: New York, 1974. Weisz, P. B. Pure Appl. Chem. 1980, 52, 2091-103. Kladnig, W. F. Acta Cient. Venea. 1975, 26, 40-69. Venuto, P. B.; Landis, P. S . Adu. Catal. 1968, 18, 259-371. ( 5 ) Parise, J. B.; Gier, T. E.; Corbin, D. R.; Cox, D. E. J . Phys. Chem., in press. (6) Parise, J. B.; Prince, E. Mater. Res. Bull. 1983, 18, 841-2. (7) Rietveld, H. M. Reactor Centrum Nederland Research Report RCN 104, unpublished, 1969a. (8) Rietveld, H. M. J . Appl. Crystallogr. 1969, 2, 65-71. (9) McDaniel, C. V.; Maher, P. K. In ”Zeolite Chemistry and Catalysis”; Rabo, J. A., Ed.; American Chemical Society: Washington, DC, 1976; ACS Monogr. No. 171, p 285.
0 1984 American Chemical Society
Parise et al.
2304 The Journal of Physical Chemistry, Vol. 88, No. 11, 1984
TABLE I: Experimental Conditions Used To Collect Data at NBS and IPNS NBS IPNS monochroma- reflection (220) of Cu flight path, 14 m tor detector position, &90° wavelength, 1.5423
20
A
-
collimation 10’-20’- 10’ 15’ arc mosaic 1-30, 21-50, 47-70, angular ranges, 69-90 deg d range, 8, 12.63-1.09 3.566-1.074 angular step, 0.05 deg sample A1 can, 16-mm vanadium can, 11.3-mm container diameter diameter discrepancy factorsQ R,, = lOO[C,w?/y(obsd) - (l/~)y(calcd))~/~,w,/y(obsd))~]’/~ Re = 100[(N - P)/C,{W$(O~S~)]*]’/’ RB = lOO[C,(lZ(obsd) - (l/c)Z(calcd)l/CZ(obsd)]
15
* A
v
0
-
: r
10
5
0
I
14.3
I
I
I
14.7 a(A)
I
-
“y(obsd) is the observed intensity (including background); w,is the statistical weight previously described (see Introduction);I(obsd) and I(ca1cd) are the observed and calculated integrated intensities, where I(obsd) is approximated according to the method of Rietveld;’,* N is the number of observations; and P is the number of refined parameters.
I
15.0
Figure 1. Predicted (line) and observed (letters) distortion of the 8-ring in zeolite Rho (after Parise et aL5). Points marked as follows: (a)
ND4-Rhol lK (deuterated ammonium-exchanged zeolite Rho at 11K); (b) D-RhollK (deuterated zeolite Rho at 11 K); (c) Cs-Rho (Cs-exchanged Rho); (d) D-Rho (deuterated Rho at 275 K); (e) CD,ODRhollK (deuterated Rho with 18% by weight of adsorbed deuterated methanol at 11 K); (f) Ca-Rho (Ca-exchanged Rho); (g) D20-Rho (hydrated Rho); a, e, and f refer to unpublished results while c refers to data in ref 6. National Laboratory, respectively. Details of the experimental conditions used are given in Table I. Both types of sample containers were designed to expose the zeolite powder to vacuum a t the temperature at which data were collected, in order to minimize the effects of any water introduced into the powder during loading in the drybox (NBS sample) or N,-flushed glovebag (IPNS). The aluminum can used at the NBS was fitted with a rubber 0 ring which froze when the sample was cooled in a closed cycle Displex (Air Products and Chemicals, Inc.) refrigerator. The vanadium can used at the IPNS was fitted with an indium plug which melted above 430 K and exposed the powder to the evacuated sample chamber of the special environment powder diffractometer (SEPD) at IPNS. After being cooled over a 12-h period, the sample of hydrogen Rho (designated D-Rho1 1K) was believed to be at approximately 11 K. This temperature may be as high as 20 K since the recording thermocouple was located in the block of the Displex unit and read 7 K. On the basis of previous experience with insulating samples, the temperature has been set to 11 K with the understanding that most of the error will be on the high side of this value. A similar situation arose in determining the temperature of samples placed in the vacuum chamber of the SEPD at the IPNS. The recording thermocouple was placed in contact with one end of the sample container. However, since the heat shields have open windows for the straight-through neutron beam, a temperature gradient can exist across the sample. Thus, the actual sample temperature may be in error by up to f 10 K. However, once the temperature was set, it was precisely controlled by a proportional controller. In an attempt to thoroughly dehydrate the samples, data were collected at the higher temperature first
(573 K) and then at the lower temperatures (423, 295 K). These three data sets are designated D-Rho573K, D-Rho423K, and D-Rho295K, respectively. The importance of adequate dehydration was apparent during the collection of the data set for D-Rho573K. Close inspection of the profiles of peaks showed that they had shoulders on the high &spacing side. This clearly indicates that the sample’s cell dimension was changing while data were being collected. Qualitatively, the constant ratio of intensity between the subsidiary peaks and shoulders to the dominant peaks in the pattern suggest that this is in fact a related second phase rather than a distortion in the cubic symmetry of zeolite Rho at 573 K. Results and Discussion Structure Refinement. Data from both NBS and IPNS were used in the Rietveld refinement of the structure of hydrogen forms of Rho. D-RhollK will be discussed first. The cell dimension determined from the neutron powder diffraction pattern (see supplementary Figure 1) was estimated to be 14.6 A. Using this dimension and an average (T-0) bond length of 1.63 A, distance-least-squares (DLS) calculations5 provided a starting model for the least-squares procedure. Full refinement of position, isotropic thermal and profile parameters (unit-cell dimension, diffractometer zero half-width, and background parameters) led to the discrepancy factors given in Table I1 and described in Table I. N o attempt was made to account for peak asymmetry, and the low angle data before 8.0’ in 20 were excluded from the refinement. Careful scrutiny of difference Fourier maps followed by least-squares refinement in an attempt to define positions for deuteron sites within the zeolite led to inconclusive results. Three sites, located at (0.11,0.11,0.54), (0.215,0.215,0.64), and (0,0,0.4) had refined populations of 1.5 (3), 0.5 (2), and 1.9 (3), respectively. Although inclusion of these three positions lowered R,, from 6.85 to 6.59, the uncertainty involved in assigning these sites to D+ or D,O+ as opposed to D20, along with their low occupancy, made any speculation on the positions of proton sites in Rho unfruitful. The final parameters listed in Table I1 are those for the final refinement excluding all extraframework species except Cs. Initial refinements in which the occupancy of this site was fixed in accordance with the chemical analysis, and the thermal parameter ( B ) was refined, gave a value of R,, of 7.24. On the other hand, fixing the isotropic thermal parameter to a value of 2.7 AZand refining the occupancy gave a value of R,,
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2305
Flexibility of the Framework of Zeolite Rho
TABLE II: Final Atomic Parameters for D-Rho1 lK, D-Rho295K, D-Rho423K, and D-Rho573K“ D-Rho1 1K D-Rh0295K D-Rh0423K ao, A m 14.601 (1) 14.7237 (5) 14.7580 (5)
D-Rh0573K 14.8680 (8)
1
0.2726 (7) 0.1197 (8) 0.4219 (6) 1.2 (2)
0.2726 (8) 0.1171 (4) 0.4183 (5) 2.9 (2)
0.2724 (1) 0.1158 (10) 0.4167 (6) 3.6 (2)
0.2674 (22) 0.1138 (13) 0.4120 (10) 3.6’
z
0.2127 (5) 0.3956 (8) 2.3 (3)
0.2085 (4) 0.3930 (7) 1.7 (3)
0.2041 (5) 0.3898 (11) 3.1 (3)
0.1935 (9) 0.3778 (19) 3.1’
z
0.1315 (5) 0.6227 (7) 1.0 (2)
0.1356 (4) 0.6222 (6) 0.3 (2)
0.1368 (5) 0.6227 (6) 0.9 (2)
0.1391 (6) 0.6263 (9) 0.9‘
0.0310 (4) 0.2098 (4) 0.3878 (5) 0.5 (2) 2.7 (1)
0.0254 (3) 0.2132 (4) 0.3875 (5) 1.0 (2) 3.8‘
0.0228 (4) 0.2132 (4) 0.3862 (5) 1.5 (2) 3.8’
0.0154 (6) 0.21 11 (7) 0.3860 (7) 1.5’ 3.8’
2.7’ 6.85 4.03 1.67 10.59 1.68
1.2 (2) 4.88 3.07 1.6 12.35 1.52
1.2 (2) 4.59 2.32 2.0 14.30 1.40
0.5 (3) 5.49 2.82 1.9 22.02 1.14
X Y Z B,
A2
01 X = Y
B, A’ 02 X=Y B, A2 03 X Y
z
B, AZ Cs (O,O,I/z)
population B, A2
RW Re XZ
4
A,c A
Standard deviations in the last digits are given in parentheses.
’Fixed during refinement. CSeeFigure 1.
equal to 6.85. This E value was arrived at by multiplying the room temperature B(Cs) value, obtained from the refinement of a fully Cs-exchanged Rho6 (3.8 A2) by the ratio of the average isotropic thermal parameters of the framework atoms at the two temperatures. Once again, because of uncertainties in the actual composition of the site at (l/z,O,O), the difference between the refined population parameter for this site (2.7 atoms) and the chemically analyzed value for Cs (1.15) is assumed to be due to the presence of D+ or D30+. Since the introduction of HzO into this site would decrease the effective population parameter, since the scattering length of Cs was used throughout the refinements described above (the overall scattering length for H 2 0 is about -0.16 fm), partial hydration of the zeolite during loading into the Al sample container appears unlikely. Throughout these refinements and those described below, Si and A1 were presumed to be randomly distributed over the 48h sites of the space group 133m (217), and they will therefore be referred to as “T sites” or “T atoms”. The scattering length of the T sites was fixed in accordance with the chemical analysis, assuming no A1 was removed from the framework upon deammoniation. Its occupancy was not refined. All structural refinements were carried out by using local modification^'^^^ of the Rietveld7v8program. Both NBSI2 and IPNS” versions differ from earlier version^^*^^^^ in that background is included as part of the model. The forms of the background parameters, along with description of the bond length, bond angle, error calculations, and Fourier synthesis programs, have been described el~ewhere.I*-’~The following scattering lengths were (10)Hewat, A. W.UK Atomic Energy Authority Research Group Report RLL 73/897,unpublished, 1973. (11) (a) Von Dreele, R. B; Jorgensen, J. D.; Windsor, C. G. J . Appl. Crystallogr. 1982,15, 581-9. (b) A description of the IPNS instrument can be found in: “ICANS-VI, Proceedings of the Sixth International Collaboration on Advanced Neutron Sources”; Argonne National Laboratory: Argonne, IL, June 28-July 2, 1982 (ANL-82-80, 1983); pp 105-14. Other software included ORTEP, an atomic structure drawing program by C. K. Johnson of Oak Ridge National Laboratory, Oak Ridge, TN, and ~ ~ 7the 6 , distance-least-squares program of Ch. Baerlocher, A. Hepp, and W. Meier of the Institut fur Kristallographier, ETH, Switzerland, based on the work of H. Villiger, D. Phil. Thesis, London University, 1969. (12) Prince, E. NBS Tech. Note (US)1980,No. 1117, 8-9. (13) Jorgensen, J. D.;Rotella, F. J. J . Appl. Crystallogr. 1982,15, 27. (14) Finger, L. W.;Prince, E. NBS Tech. Note ( U S . ) 1975, No. 854.
used: b(Si) = 4.149, b(A1) = 3.449, b ( 0 ) = 5.805, and b(Cs) = 5.42 fm.I5 The structure of the hydrogen form of Rho from 295 to 573 K will now be described. The parameters for DzO-exchanged stabilized form of Rho from a previous refinementS were used as a starting point. Although the cell parameters for these two samples are quite dissimilar (14.69 8,compared with 14.72 A for the present refinement), the convergence on the parameters shown in Table I1 was smooth and rapid. The difference in cell dimension for two samples of nominally equivalent chemical compositions5 serves to underscore the difficulty in comparing zeolites prepared from different batches or, as is the case here, calcined under different conditions. Zeolite Rho, in particular, shows great variability in unit-cell parameter depending on ion exchange, calcination temperature and conditions, hydration state, and t e m p e r a t ~ r e . ~ + ~ ,In ’ ~this , ’ ~ case, the sample having the 14.69-A cell (0.95 Cs atoms/unit cell) was probably inadequately deammoniated. Increase in cell dimension for Rho upon deammoniation is well d ~ c u m e n t e d . ~ - The ’ ~ J ~four data sets discussed here were collected on a sample which had been ammonium exchanged, deammoniated, dehydrated, and DzO exchanged in the same batch. They were separated only for loading into sample containers and are distinguished by the temperature at which data were collected and the source of neutrons (steady-state reactor at the NBS, spallation source a t the IPNS at Argonne) used for the study. The problem of the occupancy of the site normally associated with Cs (1/2,0,0) was handled in a fashion similar to that described previously, except that the thermal parameter was fixed at 3.8 A2 throughout. This was the value derived from a previous study of a fully Cs-exchanged sample of Rho.6 Using programs modified to apply the Rietveld technique to time-of-flight data,11J3we refined the structures of Rho at 295, 423, and 573 K (see supplementary Figures 2-4). The variables in the refinements were (15) Koester, L.;Rauch, H. “Summary of Neutron Scattering lengths”; IAEA contract 2517/RB, 1981. (16) Flank, W. H. “Molecular Sieves-IV”; Katzer, J. R., Ed.; American Chemical Society: Washington, DC, 1977;ACS Symp. Ser. No. 40. (17) Barrer, R. M;Barri, S.; Klinowski, J. “Proceedings of the Fifth International Conference on Zeolites”;R e s , L. V., Ed.; Heyden: London, 1980; pp 20-9.
2306 The Journal of Physical Chemistry, Vol. 88, No. 11, 1984
Parise et al.
TABLE III: Interatomic Distances (Angstroms) and Angles (degrees) for D-Rho1 lK, D-Rho295K, D-Rho423K, and D-Rho573K“ D-RhollK D-Rh0295K D-Rh0423K D-Rh0573K T-0 1 1.66 (1) 1.69 (1) 1.70 (1) 1.69 (2) T-02 1.60 (1) 1.57 (1) 1.56 (1) 1.59 (2) T-03 1.66 (1) 1.67 (1) 1.69 (1) 1.73 (2) T-03’ 1.62 (1) 1.59 (1) 1.58 (1) 1.57 (2) 1.63 1.63 1.63 1.64 (T-0) 01-02 2.66 (1) 2.66 (1) 2.67 (1) 2.69 (1) 01-03 2.66 (1) 2.70 (1) 2.68 (1) 2.66 (1) 01-03‘ 2.71 (1) 2.67 (1) 2.67 (1) 2.76 (1) 02-03 2.69 (1) 2.67 (1) 2.62 (1) 2.54 (1) 02-03’ 2.64 (1) 2.64 (1) 2.68 (1) 2.79 (1) 03-03’ 2.67 (1) 2.64 2.66 (1) 2.69 (1) 2.67 2.66 2.66 2.69 (0-0) T-T’ 3.16 (2) 3.24 (2) 3.27 (3) 3.23 (3) T-T” 3.08 (2) 3.04 (2) 3.01 (2) 3.04 (3) T-T”’ 3.02 (2) 3.04 (2) 3.05 (2) 3.07 (3) T-T”” 3.02 (2) 3.04 (2) 3.05 (2) 3.07 (3) 3.07 3.09 3.09 3.10 U-T) cs-02 3.25 (1) 3.35 (1) 3.38 (1) 3.48 (1) CS-03 3.50 (1) 3.57 (1) 3.58 (1) 3.57 (1) 01-T-02 109 (1) 109 (1) 110 (1) 110 (1) 01-03 106 (1) 107 (1) 105 (1) 102 (1) 01-03’ 111 (1) 109 (1) 109 (1) 115 (1) 02-T-03 111 (1) 111 (1) 111 (1) 114 (1) 02-T-03’ 110 (1) 113 (1) 113 (1) 107 (1) 03-T-03’ 109 (1) 108 (1) 109 (1) 109 (1) (0-T-0) 109 109 109 109 T-0 1-T 144 (1) 148 (1) 149 (1) 145 (2) T-02-T 148 (1) 150 (1) 150 (1) 145 (1) T-03-T X2 134 (1) 138 (1) 138 (1) 137 (1) (T-0-T) 140 143 144 141
“Standard deviations in the last digits are given in parentheses. the unit-cell parameter, atomic positions and isotropic temperature factors for T and oxygen atoms, Gaussian peak broadening parameter, and background parameters. Unfortunately, the presence of a second (presumably partially hydrated) phase in the data set for D-Rho573K caused serious convergence problems; this is reflected in the high estimated standard deviations for parameters dervied from it. The final refined parameters for all four data sets are shown in Table 11. The final results of all three refinements are given in Table I1 and selected interatomic distances and angles are given in Table 111. Discussion Recently5s6the variation in the structure of Rho in terms of a simple distortion parameter A (Figure l), which measures the difference between the major and minor axes of the elliptical pore openings to the truncated cubooctahedral a-cages, has been described. Further, the variation of A with a cell dimensions, determined experimentally, was compared to that obtained by DLS calculations (Figure 1). In these calculations, the distances obtained from a variety of studies of the structures of the aluminosilicates are used as observables in a least-squares refinement of atomic positional parameters which minimizes the function x2 = Ciwt[Di(obsd) - D,(calcd)l2. Here Di(calcd) is the distance calculated from the least-squares model and wi are the weights, usually empirically assigned. As was pointed out in an earlier p~blication,~ provided the weights are assigned such that w(T-0) >> w ( 0 - 0 ) > w(T-T), the variation of cell distortion (A) with cell volume is a reasonable fit (Figure 1). The (T-0) distances used in the calculations (1.62 or 1.63 A) were derived from the and (T-T) average values of many refinements, while the (0-0) distances were derived from (T-0) assuming ideal T-0-T and 0-T-0 angles set internally by the program ~ ~ 7 6The . ’ weights ~ used were w(T-0) = 1 .O,w ( 0 - 0 ) = 0.07, and w(T-T) = 0.04. A more detailed description may be found in ref 5 and 11 b. The curvature at the upper (>14.8 A) and lower (-14.6 A) limits of Figure 1 may be related to the strain introduced into the framework as Rho is either compressed or expanded beyond these
limits (see ref 5, Figure 3). Although the calculated curves serve as a guide and, in the case of zeolite Rho at least, an accurate predictive tool, they should in no way be interpreted as a replacement for experimentally determined atomic structure. The influences of multiply charged exchangeable cations, of variation in (T-0),(0-0),and (0-0) distances with experimental conditions, and of the assignment of weights for those “observables” used in the DLS are parameters which are difficult to disentangle. It is a testimony to the robustness of the technique that it apparently predicts the variations observed as precisely as its does. The structures reported here cover the range in cell dimensions from 14.6 < a < 14.9 A, and we might expect on the basis of previous work5v6that the variation in A with unit-cell dimension a. exhibited by these samples should lie close to a straight line. Outside this range, the variation is distinctly nonlinear (Figure 1). Figure 2 shows this variation, and, to a reasonable approximation, it is a straight line. As expected, the ellipticity of the pore openings decreases as the cell increases. Plotted on the same diagram are data from Cs-exchanged6 Rho at different temperatures and a deuterated sample of Rho.S The effects of Cs exchange and increasing temperature are now easily seen and are clearly separable. At room temperature, extraction of Cs from Rho causes the lattice parameter to increase and the distortion to decrease. On the other hand, an increase in temperature for both Cs- and D-Rho forms causes a decrease in the ellipticity of the pore opening. The difference between samples of deammoniated dehydrated Rho (designated D-Rho295K in this study and D-Rho in ref 5 ) , shown as points d and f i n Figure 2, require further explanation. Since D-Rho295K had been pretreated at above 420 K under vacuum for about 70 h, it is unlikely to contain more water than D-Rho, which had been loaded in a glovebag under flowing helium and not heated following this procedure. The lower cell dimension for D-Rho is, therefore, probably due to inadequate decomposition of ammonium ion following its exchange for (Na,Cs) in the assynthesized material. Corroboration for this conclusion is found in the occupancy for the site at (‘/*,O,O)which is 4.2 equivalent Cs atoms as opposed to the value of 1.15 expected from the
Flexibility of the Framework of Zeolite Rho
18
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2307
I
00o.j
CPHRHO. D R T
,
CYCLE!
,211
800.
16 c
+
500.
a a c
400.
A
-1
x 300 DEG C
I
m K a
X
a
300. 200.
t
100.
l2
.
101 14.60
*
.
1
I
14.70
.
*
1
0.
I
14.80
atA) Figure 2. Variation of A with cell dimension for (a) D-Rho1 lK, (b) Cs-RhoRTV6 (Cs-exchanged Rho, room temperature after being pretreated at 493 K), (c) Cs-RhoRT6 (as for b before heating), (d) D-RhoS (deammoniated Rho at room temperature), (e) Cs-Rho2206 (as for b and c at 493 K), (f) D-Rho295K, (g) D-Rho423K; (h) D-Rho573K.
i
f o r A Vs. T for a.Vs. T
{
, 150
,,
0
OOnN
700.
100
200
I
I
I
I
I
300
400
500
600
100
I146
000
T(I0 Figure 3. Variation of A (see Figure 1 and text) with temperature (0. The curve has been fitted to the four points by eye. The points correspond to (a) D-Rho1 lK, (b) D-Rho295K, (c) D-Rho423K, and (d) D-Rh0573K.
chemical a n a l y s i ~ .A~ study of fully ND4+-exchanged Rho'* at 1 1 K confirms that the site at (0,0,'/2) is fully occupied in this case. These results are in accord with those of FlankI6 and others; ammonium exchange causes a decrease in the cell dimension of zeolite Rho. With regard to the present study, Figure 3 plots the variation of both cell parameter (ao) and distortion parameter (A) as a function of temperature. Extrapolation of the A vs. T curve suggests that A becomes zero (the ring becomes circular and the structure centrosymmetric in space group Im3m) above 620 K. A change in symmetry from Z43m to Im3m has been reported upon dehydration of either the H form5 or the (Cs,Na) formlg (18) Parise, J. B. unpublished results.
i. d.
id.
2d.
3d.
rd.
5d.
6d.
TWO THETR
Figure 4. Sequence of X-ray powder diffraction patterns (Cu K a radiation, X = 1.5418 A) for deammoniated zeolite Rho. The spectrum at 28 OC is that of a partially hydrated sample. Note the similarity between relative peak intensities for the lower and higher temperature patterns.
and also at high temperatures (>773 K) for the ammonium form.*O An X-ray experiment was performed in which deammoniated zeolite Rho was heated from 301 K (28 "C) to 773 K (500 "C) and then allowed to cool (Figure 4). Upon heating, the zeolite initially loses water, and the cell contracts (the diffraction peaks shift toward higher values of 28). The cell then expands (lines move to lower values of 28) above 473 K (200 "C) and undergoes an apparently smooth transition up to 773 K. The spectra marked 500 and 28 O C show qualitative similarity in terms of both cell parameters (N 15 %.)and relative intensities of peaks (particularly in the region from 16' to 30° in 28). Deammoniated Rho is then centrosymmetric in the hydrated form, noncentrosymmetric upon d e h y d r a t i ~ nand , ~ centrosymmetric once more at high temperatures. Since the configuration of the elliptical pore opening changes accordingly, the temperature and other conditions of calcination (under vacuum or in air, for example) will be critical factors in preparing samples of Rho with consistent properties. These and other studies suggest that the structure of zeolite Rho is likely to be centrosymmetric above 550 K. Calcination above this temperature may be necessary to completely desorb ammonia. Acknowledgment. It is a pleasure to acknowledge Yvonne King for her patience throughout the course of this work and for typing "just one more" set of corrections. We are indebted to Glover Jones for generously giving of his time to perform the X-ray diffraction experiments at high temperature. The work at IPNS was supported by the U S . Department of Energy. Supplementary Material Available: A table of Rietveld-refinement observed and calculated profiles for D-Rho1 1K as well as supplementary Figures 1-4, which are plotted profiles for D-Rho1 lK, D-Rho295K, D-Rho423K, and D-Rho573K, respectively (8 pages). Ordering information is given on any current masthead page. ~~
(19) Baerlocher, Ch.; McCusker, L. B. In "Proceedings of the Sixth International Conference on Zeolites, Reno, 1983". (20) McCusker, L. B. In "American CrystallographicAssociation Program and Abstract"; 1983; Vol. 11, p 45, Abstract PE1.
